Concrete compositions contain aggregate (e.g., sand and rock), cement and water. When water is added to the cement, a paste forms which hardens over time to a solid structure. Various additives have been used in cement compositions to modify their properties. Long fibers such as asbestos reduce the sagging of these pastes and thus is beneficial when applying tiles to a vertical surface. Freezing point depressants are used when cements are to be poured in subfreezing temperatures.
Bentonite clay has been used for this purpose. Other polymers, such as the polyvinyl alcohols and methyl methacrylates, have been used to reduce friction when pumping these pastes and to otherwise modify their workability. Fumed silica is used as an additive to make stronger concrete with reduced permeability.
U.S. Pat. No. 4,963,668 describes cement compositions comprising 0.01 to 0.9% welan gum by weight of dry cement to improve workability, suspension of aggregates, air entrainment, sag resistance, flow characteristics and resistance to water loss.
Incorporation of fluid-enhancing polymeric material into cement-containing mixtures is difficult when the fluid-enhancing material requires hydration. Such hydration must be achieved by lowering the initial amount of water used to mix the cement and sand, hydrating the polymer with water, and then adding the polymer/water dispersion to the initial cement/sand/water mix to obtain a cement/sand/water/polymer mixture having the desired final weight ratio.
When water is held out of the concrete to hydrate the polymer, the amount of water which can be used to initially disperse cement and sand is insufficient to form a uniform mix. Large golf and bowling ball-size agglomerates form which do not disperse even when the balance of the mixing water is added. Furthermore, pre-hydration of the polymer does not occur quickly enough to stay in synchronization with the typical 60-90 second batch plant mixing cycle. It is also impractical to have a large quantity of hydrated polymer solution available for inclusion into the concrete without significant additional costs for preservation, mixing, and storage.
Addition of water containing hydrated polymer to a dry cement/sand mixture such that the desired cement/sand/water/polymer weight ratio is achieved immediately is undesirable because the polymer can interfere with early stages of cement hydration and may impair mechanical properties of the mixture. Such addition also makes dispersion of the cement particles more difficult.
The present invention is a rapidly hydrating welan gum useful for improving cement workability. The gum is an improvement over the prior art because predispersion/prehydration of the gum prior to mixing with concrete is not required in order to obtain a uniform, workable mix.